The invention relates to the field of microwave tunable devices, and in particular to microwave tunable devices on Si based wafers.
In the past few years, the use of high-permittivity ferroelectric materials such as (Ba,Sr)TiO3, SrTiO3, (Ba,Zr)TiO3, (Ba,Hf)TiO3, Bi1.5Zn1.0Nb1.5O7 and related thin films have been widely studied due to an increasing need for smaller size, light weight, higher power, and lower cost frequency agile components. It will be appreciated by those of skill in the art that BST is representative of one or more related perovskite-like tunable dielectric materials. There is a great incentive to replicate these achievements on silicon based wafers for integrated microwave device applications. If one makes BST integrated tunable circuit on Si substrate directly, mass production process can be easily realized through large size availability of Si wafers and the widespread industrial use of Si-based processing technology. However, BST films grown directly onto Si suffer from low tunability due to the formation of low-K SiO2 thin layers between BST and Si during the requisite high temperature BST deposition process. Also, the crack is easily observed on the surface of BST films.
Technically, the growth of high quality BST films on SiO2/Si can be a formidable challenge because of the inherent crystallographic incompatibility of two materials. In order to solve these problems, firstly, thick SiO2 layer more than 2 μm is required as the substrate for the minimization of microwave insertion losses of normal Si with low resistivity of 10 Ωcm is associated with loss tangent related to conductivity in the silicon substrate. Secondly, suitable oxide buffer layers are required between top BST layer and Si substrates to control the orientation and quality of the BST films.
The buffer layer between Si and BST plays a major role in determining the quality of the film and its microwave loss properties. However, oxides which can be grown epitaxially on Si substrate are limited. TiO2, MgO, LaAlO3, Al2O3, YSZ, CeO2 are, for example, possible candidates. Generally, the Si substrate introduces high microwave loss due to the low resistivity of Si.